“ a poison ivy rash (like any other allergic reaction) is caused by the body releasing the chemical histamine to the affected area as part of your immune response. Heat will stimulate the production of histamine, and although this creates an unpleasant itching in the moment, the heat will eventually deplete the affected cells of their histamine, which can provide up to 8 hours of itch relief afterwards. This can be achieved by aiming warm water at the affected area, and slowly increasing the heat to the maximum tolerable temperature until itching stops.”
https://teclabsinc.com/why-you-shouldnt-use-hot-water-on-a-p... (article title referring to not using hot water when washing off oils after initial exposure)
